1. Field of the Invention
The present invention relates to a display device in which the luminance of a display screen can be controlled in response to the brightness of a surrounding and a method of manufacturing the same.
2. Description of the Related Art
A technique for forming a thin film transistor (hereinafter referred to as a TFT) on a substrate is greatly improved, and thus the application to an active matrix display device is progressing. Conventionally, the active matrix display device utilized by TFTs using an amorphous silicon film requires a driver IC. However, TFTs using a polycrystalline silicon film can be operated with a high driver frequency, and TFTs in a pixel portion and TFTs in a driver circuit can be integrally formed on a substrate.
The active matrix display device in which the driver circuit is integrally formed on the substrate has gathered attention, because various advantages such as a cost reduction, a miniaturization of the device, and an improvement of a production yield are obtained in the case where various circuits such as a shift register and a sampling circuit are formed.
In the active matrix display device, TFTs are arranged in several tens to several millions of pixels, and a separate electrode (pixel electrode) is provided with respective TFTs. In the case of a liquid crystal display device, liquid crystal is filled between an element substrate in which the TFTs are formed and a counter substrate in which a common electrode is formed. A capacitor using the liquid crystal located between the separate electrode and the common electrode as dielectric is formed. The operation of the liquid crystal display device is as follows. That is, a voltage applied to the respective pixels is controlled by a switching function of the TFT and charges are stored in the capacitor to drive the liquid crystal. Then, an amount of light transmitted through the liquid crystal is controlled to display an image. Although there is the reflection type liquid crystal display device using external light, the liquid crystal display device with a backlight unit or a front light unit as a light source is generally used.
On the other hand, a display device in which a light emitting element is provided for respective pixels and turning of or off of the light emitting element is controlled by the TFT to display an image is developed. In this device, the light emitting element utilizes electro luminescence (hereinafter is referred to as EL). Thus, such a display device is also called an EL display device. In an active matrix EL display device using the TFTs, a TFT for switching (hereinafter is referred to as a switching TFT) is provided for respective pixels. A TFT for current control (hereinafter is referred to as a current control TFT) is operated by the switching TFT to make an EL layer (corresponding to organic compound layer including a light emitting layer) emit light. There is the EL display device described in, for example, Japanese Patent Application Laid-open No. Hei 10-189252.
Thus, even in the cases of using external light and using light by self light emitting, the active matrix display device controls a luminance of a screen with the TFTs in accordance with an input voltage based on an image signal, to display an image.
However, in many conventional display devices, an input voltage characteristic for image display is fixed, and thus sufficient attention is not paid such that a maximum luminance required for the display device is changed in response to a surrounding. In the case where the surrounding is nighttime and dark, even if the same luminance as in the case where the device is used outdoors in day is not obtained, an image to be displayed can be recognized. However, in this case, the luminance is not controlled. Thus, a user will see a glare and visibility is deteriorated in many cases.
Of course, a method of detecting the brightness of the surrounding by a sensor and then controlling the luminance of the screen is proposed. As a sensor for detecting the brightness, that is, the illuminance, a photodiode, a phototransistor, or the like is used. However, when those sensors are mounted as separate parts on the display device, a further area is required for the sensors. The external light is scattered by objects around the display device and incident into the photosensor with various angles. As a result, there is a problem that a difference is produced between the brightness of the surrounding and the luminance correction.
Also, there is a problem that, although dependent on a kind of sensor, if an optical filter is not attached to the display device in order to fit a spectral sensitive characteristic of a sensor to a luminosity of a person, an error in the correction is produced. For example, spectral sensitivity of a sensor using single crystalline silicon is extended to an infrared light region. Thus, in order to correct the brightness with accuracy, it is necessary to provide a visual sensitivity correction filter. Therefore, an enlargement of the display device cannot be prevented.